1. Field of the Invention
The present invention relates generally to an engine cooling system for outboard motors, and more particularly to such an engine cooling system which is equipped with a thermostat operable to open and close a cooling water passage depending on the temperature of cooling water inside the cooling water passage, and a relief valve operable to open and close the cooling water passage depending on the pressure of cooling water inside the cooling water passage.
2. Description of the Prior Art
A conventional engine cooling system for outboard motors is disclosed, for example, in Japanese Patent Laid-open Publication No. 8-100671.
The disclosed engine cooling system includes a cooling water supply passage and a cooling water return passage (drainage passage) that are formed by two juxtaposed recessed portions extending longitudinally in a vertically extending lateral projection on one side of a cylinder block, and a cover attached to the projection to close the recessed portions so as to define a water jacket including the cooling water supply and drainage passages. The cooling water drainage passage extends along a mating surface between the cylinder block and a cylinder head.
The cover has a vertically elongated configuration and is provided with a thermostat which opens and closes a cooling water passage depending on the temperature of cooling water inside the cooling water passage. The cover is also equipped with a relief valve which opens and closes the cooling water passage depending on the pressure of cooling water inside the cooling water passage. The thermostat and the relief valve are vertically spaced far from each other.
The engine cooling system of the foregoing construction operates such that when the pressure in the cooling water supply passage is higher than a predetermined pressure, the relief valve is open, allowing the cooling water to flow into the cooling water drainage passage; and when the cooling water temperature exceeds a predetermined temperature (thermostatopening temperature), the thermostat is caused to open, allowing the cooling water to flow into the cooling water drainage passage.
The conventional engine cooling system, however, has a drawback that when the cooling water temperature is below the predetermined thermostat-opening temperature, the cooling water does not flow through a portion of the cooling water drainage passage extending between the position of the thermostat and the position of the relief valve spaced from the thermostat position, regardless of whether or not the relief valve is open. If this condition continues, the engine wall develops a temperature difference between two remotely spaced portions of the cylinder block due to lack of cooling water in the drainage passage portion. This temperature difference makes it difficult to maintain a uniform temperature distribution over an area extending along the mating surface between the cylinder block and the cylinder head.